Has anyone ever done phase modulation by feeding back a ramp into
the linear freq input through a VCA? Control the phase by
controlling the VCA. Looks like you'll need to capacitivly couple
them to prevent the offset from changing the freq but it seems to
work. Using a pan/fade to fade between Ramp and saw instead of the
VCA allows for a wider phase modulation. This should sound like PWM
but without some of the higher harmonics which should reduce the
buzzyness. I can see it on a scope, need to find some time to patch
it up.
-----
Roger,
I have done something very similar to this. I have attached 3 links
below to short MPG movies with sound to illustrate the effect. My
implementation is different. But, my goal was waveshaping and PWM
type effects on waves other than pulse waves.
The set up is: VCO output normally to the audio input side of VCA.
output of VCA is the shown output. The control of the VCA is another
wave out of the same VCO (through a reversing attenuator). You get
varied effects depending on which wave you use for the control side
of the VCO. Of course, pulse wave is not one of the interesting
choices. In this arrangemnent, you get static control of the
waveshape via the reversing attenuator.
Now, replace the reversing attenuator with a mixer. Your control
signal for the VCA is a mixture between one of the VCO waves with
some LFO mixed in. The LFO can provide the PWM sounding effects on
the wave. It is not really PWM, but it has that sound.
Here are the movies:
http://www.wiseguysynth.com/larry/misc/Mov01.mpg
http://www.wiseguysynth.com/larry/misc/Mov02.mpg
http://www.wiseguysynth.com/larry/misc/Mov03.mpg
This general concept of waveshaping with audio out of the same VCO is
not my original idea. Ken Elhardt turned me onto this and this is my
adaptation. I am actually doing this one with a modified Blacet VCO
which has the OTA for waveshaping built into the module. I added the
reversing attenuator mixer to the module. So, aside from that VCO,
only a LFO is used to produce these waves. But, it is effectively
the patch described as above.
Larry Hendry
